The present invention relates to a DC-DC converter which pulses the DC by chopping it and then, after the voltage by a transformer, rectifies it and smoothes it to generate a DC voltage, for devices that use an insulated DC voltage in a transformer, rectifies it and smoothes it to generate a DC voltage, for devices that use a high DC voltage such as plasma display devices and electroluminescent display devices.
With the demands for small size, light weight and low power OA devices, there are similar demands for DC-DC converters.
FIG. 5 shows a prior art DC-DC converter. A primary winding 121 of a transformer 12A and a MOS transistor switch 14 are connected in series with a battery 10. This MOS transistor switch 14 is cyclically turned ON/OFF by a pulse train which is output from a control circuit. To increase the conversion efficiency by reducing the ON resistor of the MOS transistor switch 14 and to achieve high-speed switching of the MOS transistor switch 14, a power-supply voltage Vcc for the control circuit 16 is greater than a voltage Vdd of the battery 10. For example, when Vdd equals 12 V, Vcc equals 15 V.
AC, which has been induced in a secondary winding 123 of the transformer 12A, is rectified by a diode 18, smoothed by a capacitor 20 and then supplied to a power-supply voltage input terminal Vcc of the control circuit 16. Since no voltage is induced in the secondary winding 123 until the MOS transistor switch 14 is turned ON/OFF, the voltage is initially supplied to the control circuit 16 from the battery 10 via the diode 22.
The AC voltage that is induced in the secondary winding 122 of the transformer 12A is then rectified by a rectifier circuit 24A, smoothed by a smoothing circuit 26A and output as the DC voltage Vee.
When the MOS transistor switch 14 is turned OFF from the ON state, a fly-back voltage is induced to the primary winding 121 from the secondary winding 122 and a high voltage spike noise is applied to the MOS transistor. Switch 14 and the control circuit 16. To eliminate this spike noise, a snubber circuit 28A is connected to the primary winding 121. With this fly-back voltage a capacitor 281 is charged and the charge is discharged via a resistor 282A.
FIG. 6 shows another prior art DC-DC converter. In this circuit, a transformer 12 that is provided with only a single winding 122 on the secondary side and the output voltage Vee is supplied to a 3-terminal regulator 30 to generate the power-supply voltage Vcc for the control circuit 16.
However, in the circuit shown in FIG. 5, since the transformer 12A is provided with two secondary windings, the size and weight of the transformer 12A is increased.
Also, in the circuit shown in FIG. 6, as the voltage Vee is stepped down to the voltage Vcc by the 3-terminal regulator 30, power consumption of the 3-terminal regulator is considerable.